1. Field of the Invention
The present invention relates to a method of fabricating a bonded Silicon On Insulator (SOI) substrate having a highly uniform thin film.
2. Description of the Prior Art
The bonded Silicon On Insulator (SOI) technique using single crystal silicon wafers is known as a method of fabricating semiconductor thin films of high crystal quality, the back side of which is inseparably covered by a silicon oxide film as an insulator.
According to a known method employing the bonded SOI technique, a single crystal silicon wafer which is later thinned (hereinafter referred to as a bond wafer) and a single crystal or polycrystal silicon wafer which has a role as a backing (hereinafter referred to as a base wafer) are used, and at least one of these silicon wafers is oxidized to form an oxide film on a surface of the silicon wafer. The bond and base wafers are superposed one above another with the silicon oxide film disposed therebetween as forming an intermediate layer, and after that these wafers are bonded together while heating them at a predetermined temperature. Then, the silicon bond wafer is thinned into a thin film, thus fabricating a bonded SOI structure.
Since the resultant single crystal silicon thin film (SOI layer) is obtained by thinning the single crystal silicon bond wafer into a thin film, the crystal quality of the silicon film does not change from that of the original single crystal silicon bond wafer and hence is quite satisfactory for a start of electronic device fabrication.
Various thinning techniques for the single crystal silicon bond wafer are known as summarized below.
(1) A high-doped n+ or p+ silicon bond wafer with a thin light doped n- or p- epitaxial layer is oxidized and then bonded with a silicon base wafer, the surface of the epitaxial layer facing to that of the base wafer. Subsequently, a selective etching process is performed to etch away the n+ or p+ silicon bond substrate while leaving the n- or p- epitaxial layer as bonded to fabricate a SOI structure.
(2) A silicon bond wafer is thinned by grinding off with an ultra high precision surface grinding machine.
(3) A silicon bond wafer with a grown-in oxide film is bonded with a silicon base wafer, and then the bond wafer is subjected to a polishing process in which the oxide film is used as a polish stopper to obtain a SOI structure.
These known thinning techniques, however, have various drawbacks as described below.
The first-mentioned thinning technique requires an epitaxial growth process and hence is costly to perform. In addition, due to difficulties in obtaining an epitaxially grows thin film with a sharp impurity concentration profile, the thickness uniformly of the thin film left is difficult to obtain (see IEDM 85, pp. 684-687, J. B. Lasky et al.).
The second-mentioned thinning technique is difficult to provide the required thickness uniformity of the thin film and also costly to perform because it requires an auxiliary means such as the selective etching.
The last-mentioned thinning technique requires complicated process, is hardly generalized due to limitations resulting from adjustment of the structural dimensions of the grown-in oxide film and frequently tends to provide an insufficient bond (see 47the Annual Device Research Conf., IEEE, VB-1 1989).
Due to its precision insufficiency, neither thinning technique stated above has succeeded in fabricating a thin SOI film of a high thickness uniformity such as about .+-.0.01 .mu.m.
In short, the bonded SOI structure has the great advantage of perfect crystal quality. However, the conventional SOI fabrication techniques are unable to provide the desired thickness uniformity of the thin film, require complicated processes and costly to perform.